The upgrade plug part number for SLC-5/04 controller operating system is 1747-OS401. List price is $173.
You can update multiple controllers with a single upgrade plug.
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The "Series C" label can be confusing. There's both a Hardware series letter and an Operating System Series letter.
The Hardware "Series C" controllers started shipping with the "Series C" editions of the operating systems (OS302, OS401, and OS501).
When you buy the upgrade kit, you will be able to update your SLC-5/04 to OS401 Series C FRN 10 operating system, but the hardware will remain Series B.
With the 5/03 and 5/04, this is not really noticeable by the user. It is important on the SLC-5/05, where the Series C hardware also denoted a new Ethernet daughterboard.
Thanks Ken, I have the upgrade somewhere, can't find it. So need to purchase another one. I use the BTR & BTW instructions for RIO, and there is no way I'm going backwards just because a processor died (doesn't happen very often).